The invention relates to improvements in papermaking machines, and more particularly to an improved method and apparatus for a web forming section of a papermaking machine.
In technical advances in papermaking machines, one of the more important developments is in the initial web forming inasmuch as with demands for increased papermaking speeds, the overall machine cannot be operated any faster than the initial web forming dewatering section. Advances have included for the fourdrinier section, a two wire forming section for increasing the efficiency and speed of the dewatering process and turning out a web which avoids the disadvantages of a two sided resultant paper web.
Accompanying demands for high quality and increased speed in the papermaking machine, the papermaking industry is becoming increasingly concerned with lowering both the manufacturing and operating costs of the equipment. In formers this means that they must be constructed with less structure and lower operating costs and simpler components which require less electricity and energy costs. Also important from the papermaker's standpoint is that the resultant machine must provide quick and ready access for changing and replacing components such as foil blades, forming wires, and for cleaning out accumulations of dirt, slime and fibrous conglomerations which accrue on the component parts during operation.
In one type of former which has become highly commercially successful, twin looped forming wires are guided through a vertical dewatering path in substantial parallelism with the stock jet delivered at the lower end of the vertical forming run. In such equipment it is, of course, necessary to guide the forming wires at the end of the run and in certain constructions, the web is transferred to an inner forming wire and it has been essential to guide the outer forming wire over a top arm construction that is large and massive. Such construction requires the building housing the equipment to be taller and requires space over the former for a crane to pass. This also requires more maintenance and because of extending the height of the overall former, the unit tends to vibrate and lower the natural frequency of the entire former. Since it is desired to increase machine speeds in order to increase production, anything which contributes to the lowering of the natural frequency is undesirable.
A further disadvantage of existing structures has been the requirement of equipment to provide a vacuum for withdrawing water from the stock through the lower wire. Such vacuum apparatus requires fan pumps and air and water removal ducts. Vacuum glands have to be provided within the couch roll beneath the arcuate segment of the roll covered by the forming wires as the web is turned and transferred onto the lower forming wire. In addition the entire forming zone extending between the breast roll and couch roll is curved which requires accurate machining of both ceramic blade elements as well as support structure on which the blades are mounted. In this type of structure, vacuum pressure is utilized both beneath the initial forming shoe located just downstream of the breast roll as well as the downstream suction box or boxes. Both of these have been conventionally located within the inner or lower forming wire. Vacuum forming and dewatering has been successful but has required a substantial amount of equipment such as in the form of separators for separating and removing air from the water and keeping the air and water separate while removing them from the former. Also, due to the forming shoe and suction box both being disposed within the inner or lower forming wire, paper two-sidedness can become a problem unless the former is run at relatively higher speed, such as 3,000 to 4,000 feet per minute depending on the paper grade to permit the deflectors to operate to remove sufficient water to make the paper more one sided.
Vacuum water removal also requires a lot of electrical power in order to maintain the full extent of the duct work connected with the forming shoe under vacuum pressure while creating and maintaining the flow of air through the web to effect dewatering over the forming shoe.
Another problem which exists connected with vacuum dewatering is that the tendency of the subatmospheric air pressure in the chamber beneath the forming wire will either distend the forming wire away from the web or distend both the web and the forming wire under the web into the gap between adjacent dewatering blades. This tends to place limits on the vacuum applied or requires closer spacing between the blades. In any event, it places more limitations on the flexibility afforded the papermaker in operating the machine.